An optical network, such as a passive optical network (PON), often delivers voice, video and/or other data among multiple network nodes. A PON is an example of a so-called “point-to-multipoint” network. A PON may conform to any of a variety of PON standards, such as broadband PON (BPON) (ITU G.983), gigabit-capable PON (GPON) (ITU G.984), or gigabit Ethernet PON (GEPON) (IEEE 802.3). The architecture of a point-to-multipoint network commonly includes a single central device that communicates with multiple network nodes. In the example of a PON, the central device is often referred to as an optical line terminal (OLT), and the network nodes are often referred to as optical network units (ONUs) or optical network terminals (ONTs). The OLT delivers data to multiple ONUs using a common optical fiber link. Passive optical splitters and combiners enable multiple ONUs to share the common optical fiber link. The optical line terminal (OLT) transmits information downstream to the ONUs, and receives information transmitted upstream from the ONUs. Each ONU terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber or customer premises node.
In most PONs, the OLT and ONUs are synchronized in terms of timing to enable the transfer of data both downstream from the OLT to the ONUs and upstream from the ONUs to the OLT. The bandwidth of the common fiber optical link is typically partitioned into time slots, where some of the time slots are reserved for downstream data transfers while other time slots are assigned to individual ONUs for upstream data transfers. If timing is not preserved and synchronized between the ONU and the OLT, two or more of the optical network devices (e.g., OLT and/or ONTs) may attempt to transmit data at the same time, causing interference and potentially loss of data.
To preserve this synchronization between these devices, the OLT commonly transmits optical signals during times of data transfer inactivity that enable the ONUs to maintain synchronous timing with the OLT. Commonly, these optical signals that enable time synchronization (which may be referred to as “timing optical signals”) comprise a one bit followed a zero bit, where the OLT transmits the timing optical signals at a known frequency. The ONU may then derive the timing from the bit transitions of the optical timing signals during times of data transfer inactivity and thereby preserve synchronous timing with the OLT.
During times of data transfer activity, the OLT may generally transmit data downstream to ONUs or receive data upstream from ONUs that includes sufficient bit transitions that the OLTs and ONUs may maintain synchronous timing. Yet, in some instances, where the data may comprise long runs of ones or zeros, timing may be lost as there are insufficient bit transitions from which the OLTs and ONUs may derive the timing. To guard against data having long runs of ones or zeros, the OLTs and ONUs may employ a common scrambler function to scramble data in a manner that greatly reduces the occurrence of long runs of ones or zeros prior to sending this data either upstream or downstream.